Shielding for an isolation apparatus used in a microwave generator

ABSTRACT

A system for reducing radiated emissions the system including a microwave generator that supplies microwave energy at a fundamental frequency, a coaxial transmission cable that transmits microwave energy between the microwave generator and a microwave energy delivery device and an isolation apparatus connected between the microwave generator and the coaxial transmission cable. The isolation apparatus is configured to electrically isolate the coaxial transmission cable from the microwave generator and capacitively couple the microwave generator ground to the coaxial transmission cable.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.12/203,734 filed on Sep. 3, 2008, the entire contents of which areincorporated by reference herein for all purposes.

BACKGROUND

1. Technical Field

The present disclosure relates to systems and methods for performing amedical procedure, wherein the medical procedure includes the generationand safe transfer of energy from an energy source to a microwave energydelivery device. More particularly, a microwave energy delivery systemincluding an isolation apparatus is disclosed to reduce undesirableradiated emissions during the delivery of microwave energy.

2. Background of Related Art

Microwave delivery systems and ablation procedures using microwaveenergy are designed to safely deliver microwave energy to a targettissue. The equipment, the act of energy delivery or the procedures usedto deliver energy may be regulated by various governmental or industrialregulations or standards, such as, for example, FCC regulations andstandards for microwave equipment or electromagnetic compatibility (EMC)regulations and standards to ensure that the microwave equipment doesnot interfere with other electronic equipment. Industrial standards maybe related to patient safety, such as, for example, providing sufficientelectrical isolation between a generator and a patient. As such, themicrowave energy generation and transmission devices are specificallydesigned to minimize and reduce undesirable energy delivery.

One common design practice used to ensure patient safety inelectrosurgical generators is to create an isolation barrier between thegenerator and the patient. This is accomplished by isolating thegenerator output from an earth ground. Isolation barriers may be createdby various generally accepted circuits, such as, for example, atransformer or capacitors that would have a low impedance at about 60Hz. While the practice of including an isolation barrier is generallyeffective with systems delivering energy in RF frequencies, deliveringenergy with a signal in a microwave frequency provides new opportunitiesfor microwave generator and system designers.

One such opportunity for microwave generators and their system designersis that microwave generators need to pass FCC regulations for EMC whileoperating. The fundamental frequency (i.e., the frequency band of thedesirable microwave signal) is usually in an Instrumental ScientificMedical (ISM) band and is not an issue. Instead, EMC issues typicallyevolve around unintended energy discharges at frequencies outside of theIMS band, such as, for example, harmonics frequencies of the fundamentalfrequency above the ISM band.

Harmonics of the fundamental frequency may be a product of the microwavegenerator's signal generator or may be induced at various locations inthe microwave generator circuits and/or the microwave energy deliverycircuit. For example, harmonics are sometimes a product of the isolationbarrier that is intended to isolate the generator from the patient andto provide patient safety. For example, the isolation barrier in amicrowave delivery system may include the floating of the coaxial shield(i.e., the practice of not attaching the coaxial shield to the ground ofthe generator). Microwave energy may run along the shield of the coaxialcable and cause the coax cable to radiate as an antenna. This antennaaffect can cause the generator's harmonics to be amplified and fail oneor more EMC standards.

The present disclosure describes a system including an isolationapparatus to reduce undesirable EMC during the delivery of microwaveenergy.

SUMMARY

The present disclosure relates generally to a system and isolationapparatus for reducing undesirable radiated emissions during a medicalprocedure. More particularly, in one embodiment of the presentdisclosure a system includes a microwave generator that suppliesmicrowave energy at a fundamental frequency, a coaxial transmissioncable that transmits microwave energy between the microwave generatorand a microwave energy delivery device and an isolation apparatusconnected between the microwave generator and the coaxial transmissioncable. The isolation apparatus is configured to electrically isolate thecoaxial transmission cable from the microwave generator and capacitivelycouple the microwave generator ground to the coaxial transmission cable.

The isolation apparatus may further include an isolation circuit boardconfigured to electrically isolate the microwave generator and thecoaxial transmission cable while passing microwave energy therebetween.In one embodiment a ground reference shield may be connected to amicrowave generator ground reference and configured to house theisolation circuit board. In another embodiment an isolation barrier maybe positioned between the ground reference shield and the patientreference shield.

In yet another embodiment the ground reference shield and the patientreference shield may form a capacitor and capacitively couple themicrowave generator ground reference to the coaxial transmission cable.The capacitive coupling between the ground reference shield and thepatient reference shield may be adjustable. By varying the overlappingsurface area between the ground reference shield and the patientreference shield, the gap between the overlapping portions of the groundreference shield and the patient reference shield or a dielectricproperty of the isolation barrier.

In still another embodiment according to the present disclosure, anisolation apparatus includes an isolation circuit board with anisolation circuit and a shield coupling that provides isolation betweena microwave generator and a coaxial transmission cable. The isolationcircuit capacitively couples a microwave generator and a coaxialtransmission cable. The isolation circuit board passes energy at afundamental frequency between the microwave generator and the coaxialtransmission cable. The shield coupling includes a ground referenceshield connected to a ground reference of the microwave generator and apatient reference shield connected to the outer sheath of the coaxialtransmission cable. The shield coupling is configured to house theisolation circuit board. The ground reference shield and the patientreference shield are capacitively coupled and form a shield couplingcapacitor. The shield coupling capacitor provides a ground reference forthe coaxial transmission cable.

In yet another embodiment, the isolation apparatus may include anisolation barrier between the ground reference shield and the patientreference shield. The capacitive coupling between the ground referenceshield and the patient reference shield may be adjustable by varying theoverlapping surface area between the ground reference shield and thepatient reference shield, the gap between the overlapping portions ofthe ground reference shield and the patient reference shield, or adielectric property of the isolation barrier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a microwave energy deliveryincluding an isolation apparatus according to an embodiment of thepresent disclosure;

FIG. 2A is an electrical schematic of a conventional microwave energydelivery circuit;

FIG. 2B is a plot of electrical waveforms, from a conventional microwaveenergy delivery circuit, at various points of the simplified electricalschematic of FIG. 2A;

FIG. 3A is a simplified electrical schematic of a microwave energydelivery circuit including an isolation apparatus of the presentdisclosure;

FIG. 3B is a plot of electrical waveforms at various points of thesimplified electrical schematic of FIG. 3A;

FIG. 4 is a perspective view of an isolation apparatus according to anembodiment of the present disclosure;

FIG. 5 is an exploded view of the isolation apparatus of FIG. 4; and

FIG. 6 is an electrical schematic of the isolation apparatus of FIG. 4in a microwave energy delivery circuit.

DETAILED DESCRIPTION

Detailed embodiments of the present disclosure are described herein;however, it is to be understood that the disclosed embodiments aremerely exemplary of the disclosure, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present disclosure in virtually anyappropriately detailed structure.

Referring to FIG. 1, a microwave energy delivery system including amicrowave generator 100, a microwave energy delivery device 110, acoaxial transmission cable 120 and an isolation apparatus 200 employingembodiments of the present disclosure, is referenced generally asmicrowave delivery system as 10. The isolation apparatus 200 isconnected between the microwave generator 100 and the microwave energydelivery device 110. In one embodiment of the present disclosure theisolation apparatus 200 connects to the coaxial connector 100 a of themicrowave generator 100 and the coaxial transmission cable 120.Isolation apparatus 200 may also be placed at various other positions inthe microwave energy transmission circuit.

Microwave energy delivery device 110 includes coaxial transmission cable120 (i.e., a coaxial transmission cable portion 120 is permanentlyaffixed to the microwave energy delivery device 110), as illustrated inFIG. 1. Alternatively, coaxial transmission cable 120 may be separatefrom the microwave energy delivery device 110 and the isolationapparatus 200. In yet another embodiment, isolation apparatus 200 mayinclude a coaxial transmission cable portion (not shown).

In yet another embodiment, the microwave energy transmission path 125includes the transmission path of the isolation apparatus 200, thecoaxial transmission cable 120 and the handle 116 (the transmissionportion of the microwave energy delivery apparatus 110 proximal theantenna 118). The length of the microwave energy transmission path 125is related to at least one parameter of the fundamental frequency of theenergy generated by the microwave generator 100.

As illustrated in FIG. 1, microwave energy delivery device includes apercutaneous device having a sharpened tip configured to penetratetissue. Isolation apparatus 200 may also be used with a catheterinsertable microwave energy delivery device, a skin surface treatmentmicrowave energy delivery device and a deployable microwave energydelivery device or other suitable device configured to deliverymicrowave energy to tissue 180.

FIG. 2A is an electrical schematic of a conventional microwave energydelivery circuit 20 without an isolation apparatus of the presentdisclosure. The circuit 20 includes a microwave energy source “VRF”, agenerator isolation device 130 (i.e., a transformer), and an electricalload 120 (i.e., a coaxial transmission cable 120 connected to amicrowave energy delivery device (not shown)). In FIG. 2A, and asdescribed herein, transformer 130 is shown merely as an example of asuitable generator isolation device. Generator isolation device 130 maybe any suitable device that transfers energy from a first electricalcircuit (microwave energy source VRF) to a second electrical circuit(electrical load 120) without direct electrical contact, such as, forexample, by inductive coupling, capacitive coupling or antenna toantenna energy transfer (wireless).

FIG. 2B is a plot of electrical waveforms at various points in thesimplified electrical schematic of FIG. 2A. The microwave generatorgenerates the signal VRF that is applied to the primary side “P” of thegenerator isolation device 130 with general characteristics of apeak-to-peak amplitude, a phase and a fundamental frequency. VRF isreferenced to ground “G” and is transformed across the generatorisolation device 130 to the secondary side “S” of the generatorisolation device 130 thereby creating a signal at “V1” and “V2” of thesecond electrical circuit. V1 and V2 have the same fundamental frequencyof VRF and related by the formula:VRF=(V1−V2)/ID _(Eff).wherein the constant “ID”_(Eff) accounts for system losses in thecircuit 20. The peak-to-peak amplitude of each of V1 and V2 is abouthalf the peak-to-peak amplitude of VRF.

An ungrounded coaxial transmission cable 130 attached to the secondary Sof the isolation device 130 carries half of the voltage on the innerconductor 122 and half of the voltage on the outer sheath 124, asillustrated in FIGS. 2A and 2B. This voltage signal V2 applied to theouter sheath 124 may cause energy to radiate from the coaxialtransmission cable 120 thereby producing unwanted and excess radiation.In addition, carrying this signal V2 on the outer sheath 124 may resultin the generation of standing waves and the generation of unwantedharmonics of the fundamental frequency. As such, the microwave generator100, the transmission path 125 or the microwave energy delivery device110 of FIG. 1 may fail radiating limits set by the FCC and may alsoresult in undesirable heating of material or tissue in contact with theouter sheath 124.

FIG. 3A is an electrical schematic of a microwave energy deliverycircuit 30 with an isolation apparatus 200 according to one embodimentof the present disclosure. The circuit includes a microwave energysource VRF, a generator isolation device 130 (i.e., a transformer), andan electrical load 120 (i.e., a coaxial transmission cable 120 connectedto a microwave energy delivery device (not shown)) and an isolationapparatus 200. Isolation apparatus 200 includes a circuit exhibiting theproperties of the present disclosure as described herewithin and isillustrated in the schematic as “C1”. The capacitance values andproperties of the circuit C1 in the isolation apparatus 200 issufficiently sized such that the circuit C1 has a low impedance at thefundamental frequency of the microwave generator 100 and a highimpedance at low frequencies.

With the isolation apparatus 200 in the circuit 30, the secondary side Sat V2 at the fundamental frequency is capacitive coupled to ground G.FIG. 3B is a plot of the electrical waveforms at VRF and V1. V2 is atground potential G and is therefore not illustrated in FIG. 3B. V1 is180° out of phase relative to VRF and the magnitude is related by theformula:VRF=V1/ID _(Eff)wherein the constant “ID”_(Eff) accounts for system losses in thecircuit 30. As such, the peak-to-peak amplitude of each of V1 isapproximately equal to the peak-to-peak amplitude of VRF and themajority of the microwave signal is carried on the inner conductor 122of the coaxial transmission cable 120.

The isolation apparatus 200 provides an AC reference point to groundpotential for the coaxial outer sheath 124 thus reducing the radiatedsignal of the coaxial transmission cable. V2 is capacitively coupled toground potential G and the voltage at V2 is substantially zero.

FIGS. 4 and 5 are perspective views of an isolation apparatus 200according to an embodiment of the present disclosure. Isolationapparatus 200 includes a ground reference shield 240, an isolationapparatus circuit board 245, a shield connector 250, a generator sideconnector 265 and a patient reference shield 270.

Ground reference shield 240 may include an upper shield 240 a and alower shield 240 b connected at one or more positions. Upper and lowershields 240 a, 240 b may be formed of a suitable conductive materialcapable of forming a capacitive relationship with the patient referenceshield 270. The capacitive relationship between the ground referenceshield 240 and the patient reference shield 270 is described in moredetail hereinbelow.

Upper and lower shields 240 a, 240 b may be connected by one or moremechanical connectors 240 c, such as, for example, pins, rivets,fasteners, screws or bolts, or by a suitable connection, such as, forexample, a compression connection a hinge connection, a welded or pressfit connection. Alternatively, upper and lower shields 240 a, 240 b mayhave a combination of connection means, such as, for example, a hingeconnection on a side and a locking mechanism or connector on a secondside. Any suitable assembly may be used provided the ground referenceshield 240 and the patient reference shield 270 form a desirablecapacitive relationship therebetween.

Upper and lower shields 240 a, 240 b are in electrical communicationwith each other. As illustrated in FIG. 4, mechanical connection 240 cmay provide a suitable electrical connection between the upper and lowershields 240 a, 240 b. In another embodiment, upper and lower shields 240a, 240 b may be electrically connected via the generator side connector265.

Patient reference shield 270 is connected to the shield connector 250 bya suitable connector, such as, for example, a threaded shield connectorattachment nut 260. Any other suitable connection may be used, such as,for example, a press-fit connection, a slot-fit connection, a lockingconnection or a welded connection.

Patient reference shield 270, shield connector 250 and the outer sheath224 of the coaxial transmission cable 220 are in electricalcommunication with each other. Attachment nut 260 may provide a suitableconnection between the patient reference shield 270 and the shieldconnector 250. Outer sheath 224 of the coaxial transmission cable 220may connect to the shield connector 250 by a suitable connection, suchas, for example, a threaded connection or a press-slip connection. Anyother suitable connection may be used provided that it provides suitableelectrical contact between the shield connector 250, the patientreference shield 270 and the outer sheath 224.

Patient reference shield 270 is configured to at least partiallysurround at least a portion of the ground reference shield 240 forming acapacitance gap there between. Gap may be controlled by the thickness ofan isolation barrier 275 positioned between the patient reference shield270 and the ground reference shield 240.

Isolation barrier 275 may be configured as a layer (or laminate) placedadjacent to or formed on one or more surfaces of the patient referenceshield 270 and/or the ground reference shield 240. For example, theisolation barrier 275 may be a dielectric paper, such as a dielectricpaper sold by DuPont under the trademark NOMEX®. Dielectric paper may beapplied to or positioned adjacent the inner surface of the patientreference shield 270 prior to or during assembly. After assembly, thedielectric paper provides a minimum separation or spacing between theinner surface of the patient reference shield 270 and the outer surfaceof the ground reference shield 240.

Isolation barrier 275 may be a laminate such as, for example anorganic-ceramic laminate sold by TACONIC under the product line of RF-35High Performance Laminates. RF-35 provides suitable peel strength, lowmoisture absorption and a low dissipation factor thereby minimizingphase shift with frequency. RF-35 may include woven fabric and ceramicsand may be coated on one or more surfaces of the isolation apparatus.

In yet another embodiment the isolation barrier 275 may be air. Aseparation distance between the inner surface of the patient referenceshield 270 and the outer surface of the ground reference shield 240 maybe maintained by a plurality of insulating offsets (not shown) thatprovide a desirable separation distance.

The various properties of the isolation apparatus 200 depend on theconductive relationship between the patient reference shield 270 and theground reference shield 240. The patient reference shield 270 and theground reference shield 240, separated by a minimal separation distance,form a parallel plate capacitor wherein the capacitance is proportionalto the area of opposing shield 240, 270 surfaces and the permeability ofthe isolation barrier 275 and inversely proportional to the distancebetween the shields 240, 270.

The capacitance of a parallel-plate capacitor is equal to:Capacitance=(∈×A)/dwherein “∈” is the permittivity of the isolation barrier 275, “A” is thearea of the opposing shields 240, 270 and “d” is the spacing between theshields 240, 270.

As such, a desired capacitance may be obtained by varying one or more ofthe area of overlapping surfaces, the dielectric properties of theisolation barrier 275, and the gap between the two opposing shields 240,270.

In yet another embodiment of the present disclosure the capacitance ofthe isolation apparatus 200 may be adjustable. In one embodiment, a gapadjustment mechanism (not shown) may vary the position of the groundreference shield 240 relative to the patient reference shield 270thereby increasing or decreasing the gap therebetween. Gap adjustmentmechanism (not shown) may change the gap dynamically or manually. Adynamic adjustment may be necessary if the microwave generator variesthe fundamental frequency during energy delivery. A manual adjustmentmay be used to calibrate the isolation apparatus 200 during assembly.

Capacitance of the isolation apparatus 200 may be adjusted by varyingthe overlap between the ground reference shield 240 and the patientreference shield 270. Overlap adjustment mechanism (not shown) mayreposition the shields 240, 270 relative to each other eitherdynamically or manually.

Capacitance of the isolation apparatus 200 may be adjusted by changingthe dielectric properties of the isolation barrier 275 or by changingthe type of material used for the isolation barrier.

Isolation circuit board 245 is housed within the ground reference shield240 of the isolation apparatus 200. Isolation circuit board 245 mayinclude a circuit configured to provide isolation between a microwavegenerator (not shown) and a coaxial transmission cable 220, as discussedhereinabove.

FIG. 6 is an electrical schematic of the isolation apparatus of FIG. 4and the microwave energy delivery system of FIG. 1. The adjacentsurfaces of the ground reference shield 240, connected to the generatorside connector 265, and the patient reference shield 270, connected tothe coaxial sheath 224, form the shield coupling capacitor “SC1”.Isolation circuit board 245 includes first and second isolationcapacitors “C1” and “C2”, respectively, that provide electricalisolation, as discussed herein above, between the microwave generator100 and the coaxial transmission cable 220.

In use, a microwave signal is supplied to the generator side connector265. The inner conductor 265 a of the microwave generator connector 265connects to the first isolation capacitor C1. The outer conductor 265 bof the microwave generator connector 265 connects to the secondisolation capacitor C2 and to the ground reference shield 240 of theshield coupling capacitor SC1. At the fundamental frequency of themicrowave energy delivery system the first and second isolationcapacitor C1, C2 appear as short circuits and pass the signal at thefundamental frequency to the inner conductor 250 a and the outerconductor 250 b, respectively, of the shield connector 250 and to theinner conductor 222 and the outer sheath 224 of the coaxial transmissioncable 220. The patient reference shield 270, connected to the outersheath 224 of the coaxial transmission cable, and the ground referenceshield 240 form the shield coupling capacitor SC1 thereby providing aground reference for the coaxial transmission cable 220.

As various changes could be made in the above constructions withoutdeparting from the scope of the disclosure, it is intended that allmatter contained in the above description shall be interpreted asillustrative and not in a limiting sense. It will be seen that severalobjects of the disclosure are achieved and other advantageous resultsattained, as defined by the scope of the following claims.

What is claimed is:
 1. A system for reducing radiated emissions, thesystem comprising: a microwave generator that supplies microwave energyat a fundamental frequency; a coaxial transmission cable that transmitsmicrowave energy between the microwave generator and a microwave energydelivery device; and an isolation apparatus connected between themicrowave generator and the coaxial transmission cable, the isolationapparatus configured to electrically isolate the coaxial transmissioncable from the microwave generator, the isolation apparatus including:an isolation circuit board configured to electrically isolate themicrowave generator and the coaxial transmission cable while passingmicrowave energy therebetween; a ground reference shield connected to amicrowave generator ground reference and configured to house theisolation circuit board; and a patient reference shield at leastpartially surrounding the ground reference shield and forming acapacitive relationship therebetween, the ground reference shield andthe patient reference shield forming a capacitor that capacitivelycouples the microwave generator ground reference to the coaxialtransmission cable; wherein the isolation apparatus capacitively couplesthe microwave generator ground reference to the coaxial transmissioncable.
 2. The system according to claim 1, wherein the isolationapparatus further includes an isolation barrier disposed between theground reference shield and the patient reference shield.
 3. The systemaccording to claim 1, wherein the ground reference shield includes: anupper shield; and a lower shield in electrical communication with theupper shield.
 4. The system according to claim 3, wherein the isolationcircuit board is disposed between the upper shield and the lower shield.5. The system according to claim 3, wherein the isolation apparatusfurther includes a generator side connector connected to the uppershield and the lower shield.
 6. The system according to claim 5, whereinthe generator side connector includes an inner conductor and an outerconductor.
 7. The system according to claim 6, wherein the isolationcircuit board includes a first isolation capacitor connected to theinner conductor of the generator side connector and a second isolationcapacitor connected to the outer conductor of the generator sideconnector.
 8. The system according to claim 1, wherein the isolationapparatus further includes a shield connector coupled to the patientreference shield.
 9. The system according to claim 8, wherein thecoaxial transmission cable includes an outer sheath coupled to theshield connector.